Knowing what´s happening in the EU pvp server regarding the ports,
lots and lots of people from different time zones are playing in a EU time base server, they conquer the ports with relative ease and given the timings are quite hard to regain. I see 2 possibilities that would ease the sovereignty changes once a place has been captured
1st Rotating windows: you conquer a port, you set your time and you start with that window of conquest, it remains with it for 5/6 days after that it moves 1 hour clockwise or counterclockwise each 2 days so at some point you can actually take it back
2nd Buy windows: the window is set but adding 5000 gold to the flag in shallow and 10000 in deep ports you can move the window 1 hour, that means x ex that you have a shallow port that you want to conquest, the flag cost 80k gold and is set to 6am and you want to conquer it at 6pm, that´s 12 hours * 5000 making a total of 140k gold expense. This one is also helping the server to get some gold back and help controlling inflation of the currency.